Delay lines for travelling wave tubes



April 14, 1959 cs. MOURIER DELAY LINES FOR TRAVELLING WAVE TUBES 3Sheets-Sheet 1 Filed Nov. '17. 1955 April 14, 1959 G. MOURKER DELAYLINES FOR TRAVELLING WAVE TUBES 5' Sheets-Shet 2 Filegi NOV. 17, 1955April 14,1959

s MOURIER 2,882,440 I DELAY LINES FOR TRAVELLINGWAVE TUBES Fild Nov. 17,1955 3 Sheets-Sheet 3 United Sttes Patent DELAY LINES FOR TRAVELLINGWAVE TUBES Georges Mourier, Paris, France, assignor to Compa gnieGenerale de Telegraphic Sans Fil, a corporation of France ApplicationNovember 17, 1955, Serial No. 547,491

Claims priority, application France November 29, 1954 9 Claims. (Cl.SIS-3.5)

The present invention relates to a delay line suitable, moreparticularly, for rectilinear travelling wave tubes, of cylindricalsymmetry. Tubes of this type, in which helically shaped delay lines areoften used, have a wide frequency band and a high gain per unit oflength.

Such delay lines are, however, unsuitable for high power tubes. As isknown, the helix of a travelling wave tube generally comprises a long,thin conductor which, in practice, is difficult to mount and cool.Consequently, a tubular metal envelope is used, coaxial with the helix,the latter being secured in said envelope by means of rods spaced alongthe length of the helix. Such an arrangement considerably reduces thepassband of the tube.

The present invention has for its object a new type of delay line fortravelling wave tubes which is of sturdier construction and is easier tocool than a helix of similar overall size and performance.

The delay line in accordance with the present invention comprisessubstantially: a tubular metal member of circular cross-section, and anarray of delay elements located in successive cross-sectional planes ofsaid member, each of said elements comprising two rectilinear andcolinear portions fixed to the tubular member, and a third portionjoining the first two portions, said third portion being in the shape ofthe arc of a circle coaxial with the tube, the third portions ofsuccessive element being located alternately on either side of theaerial plane of symmetry of said member and array.

Preferably, these elements are tubular, the cooling of the tube beingensured by a fluid flowing within said elements.

The invention will be better understood with the aid of the annexeddrawings, wherein:

Fig. 1 depicts a longitudinal section and Fig. 2 a transversecross-section through a plane which includes a delay line element of anamplifying travelling wave tube, without crossed magnetic field, andincorporating a delay line according to the invention;

Fig. 3 is a longitudinal cross-section of an amplifying travelling wavetube, with crossed electric and magnetic fields fitted with a line suchas represented in Figures 1 and 2;

Fig. 4 shows the dispersion curve of said line;

Figs. 5 and 6 show a transverse cross-section of two other embodimentsof a delay line according to the invention.

The tube of Fig. l, the principle of which is well known, comprises acylindrical metal envelope 1, one end of which carries a collectorelectrode 2 and the other end an insulated base 3. The latter supportsan emissive cathode 4, a heating filament 5, fed by conductors 6, and afocusing electrode 7 designed to concentrate the electrons in a tubularbeam 8, an accelerating anode 9 is connected to envelope 1, said anodehaving an annular aperture 22 for the passage of the beam. Supply leads10 are connected to the cathode 4 and the focusing electrode 7. A sourceof power 11 enables envelope 1 to be brought to a high positivepotential with respect to the cathode.

Within tube 1 are arranged in equidistant cross-sectional planes, anarray of elements 12, the shape of which will be described hereafter,constituting together with envelope 1 the delay line of the tube. 011the gun side of the tube, the line is coupled to an input circuit by acoaxial line 13, and on the collector side, to an output circuit, by acoaxial line 14. A winding or coil 15 fed from a direct current source,not shown, establishes an axial magnetic beam focusing field. Envelope 1is provided with a cooling jacket 16, an inlet 17 and outlet 18 for thecirculation of a cooling fluid.

As shown in Fig. 2, each element 12 consists of two substantiallyrectilinear and colinear portions AB and CD, welded or otherwise fixedto the wall of tube 1 and extending along a portion of the diameter ofthe tube, and of a semi-circular portion BEC coaxial with the tubestructure. The total length ABECD is substantially equal to where A isthe minimum wavelength of the operating band of the tube. Located oneither side of each element 12 are two further elements which aresuperposable to said first element by a movement of translation followedby a rotation of about the axis of the system. Elements 12 enclose,about said axis, a cylindrical space through which the beam 8 passes.Elements 12 are tubular defining a hollow interior communicating withspace for the coolant bounded by jacket 16, thus efficiently cooling theline. The operation of such a tube is well known and will not bedescribed here.

Fig. 3 represents an amplifying tube with crossed electrio and magneticfields, comprising a delay line similar to that of the preceding tube.Said tube comprises substantially the same elements as those of Fig. 1,these elements being designated by like references. The focusing coil 15has, however, been omitted. Further, a cylindrical electrode 19 extendsalong the axis of the tube through the insulated base 3; a ring 20 ofinsulating material serves to insulate said electrode from collector 12.

Source 11 is connected between conductor 19 and en-.

velope 1. Further, a strong electric current is made to flow in saidconductor from source 21 connected to both its ends. Two fields are thusgenerated in the space included between electrodes 1 and 19: a magneticfield produced by the current flowing in electrode'19, the circularlines of force of said field being coaxial with the tube, and anelectric field produced by the potential dilference between electrodes 1and 19, the lines of force of said electric field being radial. Theoperation of such a tube has been described in the copending applicationSer. No. 144,501.

Fig. 4 shows the dispersion curve of the lines depicted in Figures 1 and3, i.e. the variation of the delay ratio (0 being the velocity of lightand v the phase velocity of a wave on the line, the free spacewavelength of which is x) as a function of A. It is seen that betweenthe limits A (corresponding substantially to twice the length of anelement 12) and A (which, as experience shows differs from A in theorder of 20%), the value remains substantially constant at a value closeto 3 which is equal-to (,9 being the pitch of the line shown in Figure1). The pitch of a delay line according to the invention is the distancebetween two successive delaying elements, located on the same side ofthe axialplane of symmetry of the member 1. For waves longer than theslope of the curve increases rapidly and the curve attains the value fora wavelength equal to. A The lineiscalled anegative dispersion line.Such lines are particularly suitable for use in amplifier capable ofoperating in the bandwidth 71 i.e. of a comparatively wide band of about20% for instance. Both conditions of sturdy construction and adequatecooling have thus been taken care of in this new design, together with awide passband and a gain comparable to that of a tube incorporating ahelix.

Figs. and 6 show two other embodiments difiering from that of Fig. 2 inthat portions AB and CD extend along parallel chords in thecross-section of said tube. In Fig. 5, the arc BBC is lessthan asemi-circle whereas in Fig. 6 it is more than a semi-circle. In each ofthese two embodiments, the total length of each element is still equalto A being the minimum operating wavelength of the tube. In comparisonwith the embodiment of Fig. 2, the embodiment of Fig. 5 ensures, undersimilar conditions, a narrower band and a higher gain, while, on thecontrary, that of Fig. 6 ensures a wider band and a lower gain.

What is claimed is:

1. In a travelling wave amplifier tube, a delay line comprising a hollowcylindrical tubular member of an electrically conductive material and anarray of delaying elements located respectively in cross-sectionalplanes of said member, each element comprising first and secondrectilinear and colinear portions, and a third portion joining saidfirst and second portions, said third portion being inthe shape of anarc of a circle coaxial with said member, the third portion ofsuccessive elements being located alternately on either side of theaxial plane of symmetry of said member and array.

2. Delay line according to claim 1, in which said delaying elementsaremade of metallic tubes having a hollow, interior communicating with theoutside of said member.

3. Delay line according to claim 2, further comprising a coolant jacketsurrounding said member, means being provided for causing a coolantfluid to flow in the space comprised between said jacket and saidmember, and in said hollow interior of said delaying elements.

4. Delay line according to claim 1, in which first and secondrectilinear portions of each delaying element extend along a diameter ofcross-sections of said member, said third portion extending along asemi-circle coaxial with said member.

5. Delay line according to claim 1, in which said are of circle is morethan a semi-circle.

6. Delay line according to claim 1, in which said arc of circle is lessthan a semi-circle.

7. In a travelling wave tube amplifier of the type defining an electronpath having an axial direction between an electron-emitting means and acollecting means, a delay line comprising a hollow member of anelectrically conductive material disposed about said path and an arrayof delaying elements located respectively in cross sectional planes ofsaid member, each element comprising first and second essentiallyrectilinear portions and a third portion joining said first and secondportions, said third portion being essentially in the shape of an arc ofa circle coaxial with said axis, the third portion of successiveelements being located alternately on either side of the axial plane ofsymmetry of said member and array.

8. In a travelling Wave tube the combination according to claim 7wherein said tube is of the type having crossed electric and magneticfields, further comprising means adapted to produce said crossedelectric and magnetic fields.

9. In a travelling wave amplifier tube the combination according toclaim 8 wherein said means for producing said electric field includes anelectrode extending along said axis.

References Cited in the file of this patent UNITED STATES PATENTS2,678,407 Brown et al. May 11, 1954 2,708,236 Pierce May 10, 19552,728,029 Finger Dec. 20, 1955 2,746,036 Walker May 15, 1956 2,800,605Marchese July 23, 1957 2,802,135 Dodds Aug. 6, 1957 FOREIGN PATENTS730,687 Great Britain May 25, 1955

